British Columbia
The Pacific Northwest coast of North America has been accreted over millions of years by the addition of exotic terranes formed elsewhere and tacked onto the continent by plate tectonic movements.
Looking down, just off British Columbia’s southwest coast and north of Washington’s Olympic Peninsula, you can see a large, elongated, mountainous island: Canada’s breathtaking Vancouver Island. While it is one of the largest islands in the Pacific Ocean, the term “Pacific island” tends to conjure up images of remote, tropical paradises—a description that does not match today’s Vancouver Island. However, once upon a time, this landmass was indeed situated in the tropics, near the equator.
Around 380 million years ago, the oldest rocks on Vancouver Island were formed by underwater volcanic eruptions that produced piles of basalt near the equator, in what is now the middle of the Pacific Ocean. Later, shells of marine organisms piled up on the basalt layers, forming a thick shelf of limestone, which was then raised above the waves by more volcanism. This landmass, known as Wrangellia, then began drifting slowly northeast on the back of the Kula oceanic plate.
Driven by tectonic forces, Wrangellia and the Kula Plate drifted north and east until around 100 million years ago, when they collided with the west coast of North America. The Kula Plate began subducting under the North American Plate. As the raised topography of Wrangellia butted up against the edge of the North American continent, the landmasses began warping and uplifting under the tremendous strain, forming the Insular Mountains on Vancouver Island.
Vancouver Island is separated from mainland Canada by the Salish Sea and from the United States by Juan de Fuca Strait (which is actually part of the Salish Sea’s network of waterways).
Over the next 50 million years, several smaller landmasses collided with Wrangellia and joined the landmass that is now Vancouver Island. More marine basalts, granite, and gneiss were added to the mix of rocks found on the island.
Today, the Kula Plate has completely subducted under the North American Plate, but the strain generated by its descent, plus the ongoing subduction of the Farallon and Juan de Fuca Plates, still occasionally rock the island. On January 26, 1700, the Cascadia megathrust earthquake, with an estimated magnitude of 9.0, struck off the coast of Vancouver Island. In 1946, a magnitude 7.3 quake shook the Forbidden Plateau on the east side of Vancouver Island, the largest on-land quake to strike Canada in recorded history. Such quakes have left a record of landslides and collapsed mountain summits across Vancouver Island, adding more complexity to the island’s mishmash of geologic layers.
Not all plate movement under Vancouver Island is violent, however. Sometimes creep along plate boundaries occurs imperceptibly, in the form of episodic tremors and aseismic slip events. This “quiet quake” phenomenon was discovered in the early 2000s on Vancouver Island, after geoscientists at the Geological Survey of Canada noticed GPS signals were shifting on the island without accompanying earthquake activity. They found a two-week-long episode of tremors too small to be detected by most instruments. The tremors produced smooth movement along the plate boundary that in a not-so-smooth scenario would have been equivalent to a magnitude 7 earthquake. But there was no shaking or damage.
The large, heavily forested island, with snow in winter atop its interior mountains, is nestled close to British Columbia’s southwest shoreline and Washington’s northwest corner. Flights to Vancouver, British Columbia, or Seattle, Washington, may offer aerial views.
